During the past 140 years the average quantity of fresh water in the Waddenzee has doubled. This is due to a strong increase in the water supply from the IJsselmeer. The river IJssel in particular is transporting increasingly more water. Over the last 70 years the discharge from the IJssel has in fact doubled. This is due to human intervention in the course of the rivers Waal, Nederrijn and IJssel. One of the aims of this has been to improve the navigability of the IJssel.
Since the 1970s, the Netherlands Institute for Sea Research has not only measured the salinity, but also the water temperature, the quantity of plankton and the species composition of plankton. The measurements are carried out in the western part of the Waddenzee, in the Marsdiep. The KNMI [Netherlands Meteorological Institute] has carried out systematic measurements of the salinity there since 1861.
With the greater influx of river water the Waddenzee is also been supplied with more nutrients, in particular nitrate and phosphate. This is further increased by the fact that the rivers themselves contain more nutrients. Furthermore, the self-cleaning capacity of the IJsselmeer has decreased. In the 1950s about 70% of the nitrate compounds entering the lake were decomposed. Now this is scarcely 50%. The plankton in the IJsselmeer are no longer able to clean up the large quantities of nitrate. This is in part due to the fact that the volume of water in the IJsselmeer has decreased since the closing off of the Markermeer.
The measurements in the Marsdiep reveal that in particular the quantity of nitrate and to a lesser extent the quantity of phosphate have i
Contact: Michel Philippens
Netherlands Organization for Scientific Research